1. Field of the Invention
The present invention relates to a wear resistant member for electronic equipment in which deficiency due to static electricity is lessened, and a bearing and spindle motor therewith.
2. Description of the Related Art
In recent years, magnetic recording devices such as hard disk drives (HDDS) and floppy disk drives (FDDs), optical disk devices such as CD-ROMs and DVDs, and various kinds of game machines have made a remarkable development. In electronic equipment, usually a rotational driver such as spindle motor rotates a rotation axis with high speed, thereby facilitating various kinds of disks mounted to this rotation axis to function.
So far, for bearing member supporting such rotation axis, in particular, for bearing balls, metal such as bearing steel or the like has been used. However, metal such as bearing steel or the like is poor in wear resistance. Accordingly, in the field requiring a high speed rotation of more than 5000 rpm such as for instance electronic equipment, due to an increase of fluctuation of life, there are problems that reliable rotational drive cannot be provided.
In order to overcome such deficiencies, recently for the bearing balls, ceramics such as silicon nitride sintered body has been employed (cf. Japanese Patent Laid-Open Application No. JP-A 2000-314426). Among the ceramics, the silicon nitride sintered body is excellent in sliding properties and has excellent wear resistance. Accordingly, even when rotating at high speeds, mechanically reliable rotational drive can be provided.
However, a silicon nitride bearing ball is electrically an insulator. Accordingly, there occur a problem that static electricity generated when rotating at high speeds can be skillfully guided neither to the rotation axis consisting of metallic material such as bearing steel or the like nor to bearing member other than the bearing ball such as ball receiver. Thus, when without successfully dissipating to build up unnecessarily the static electricity in the bearing and periphery components, in a recording device using magnetic signals such as for instance HDD, an adverse affect may be caused on a recording medium. As a result, there are concerns that the content recorded in the HDD may be lost, and furthermore the electronic equipment itself may be destroyed.
Furthermore, portable personal computers, electronic notebooks, various kinds of mobile products and so on have been downsized year by year, accordingly for the HDDs therefor, demands for larger storage capacity and smaller size have become stronger year by year. In order to come up to such demands, for instance in the HDDs, rotation of still higher speed has been studied, and in the future, the high speed rotation of approximately 10000 rpm or more is expected to be realized. It is the bearing configured of the rotation axis, a bearing ball and a ball receiver that supports such high speed rotation, excess pressure being essentially concentrated on the bearing balls.
In addition to this, when rotating at high speeds, due to sliding of the bearing ball, a large amount of heat is liberated. At that time, silicon nitride is largely different in linear expansion coefficient from that of bearing steel (for instance SUSJ2) that constitutes the rotation axis and ball receiver. Accordingly, there occur such problems that due to the deformation caused by thermal expansion of the metal member, noise or unusual sound is generated, or due to thermal strain caused by thermal expansion, non-synchronized deflection is generated. Also from such viewpoints, the existing silicon nitride bearing ball cannot sufficiently come up to the longer time high speed rotation.
To the problems caused due to the high speed rotation as mentioned above, a bearing that employs a bearing ball consisting of a zirconium oxide sintered body of which linear expansion coefficient is close to that of metal is proposed (Japanese Patent Laid-Open Application No. JP-A 11-223220). The zirconium oxide sintered body, while, in addition to the above, being superior to the silicon nitride sintered body in fracture toughness, is an electrical insulator similarly with the silicon nitride bearing ball. Accordingly, when rotating at high speeds, there is similarly accumulated static electricity. It cannot be overcome that based on the static electricity, there occur various problems.
A conductive zirconium oxide sintered body of which specific resistance is approximately 10xe2x88x925 xcexa9xc2x7m is known (cf. Japanese Patent Publication Gazette Nos. JP-B2 1-24747 and JP-B2 3-51667). To such conductive zirconium oxide sintered body that is mainly used in cutting tools and electrical resistance heating elements, a large amount of conductivity enhancer such as carbides or the like is added to improve conductivity (low specific resistance). In the zirconium oxide sintered body wherein a large amount of the conductivity enhancer is added, while the specific resistance itself becomes lower, the conductivity enhancer that are excessively added tend to coagulate themselves. This causes to lower mechanical strength and sliding properties of the zirconium oxide sintered body.
For instance, as in the case of bearing balls, in the use always exposed to repetition fatigue of compressive stress and/or tensile stress, when the aforementioned agglomerated particle exists in a large amount, cracks tend to start therefrom. As a result, the sliding properties are largely deteriorated. Accordingly, the conductive zirconium oxide sintered body such as mentioned in the above publications is far from applying in the wear resistant member such as bearing balls. The above publications, not assuming the application of the conductive zirconium oxide sintered body in the sliding member, give the conductivity only to make use electric discharge machining.
In Japanese Patent Laid-Open Application No. JP-A 8-296649, it is disclosed that ceramic material having volume resistivity of 104 xcexa9xc2x7m or less is used in a dynamic pressure bearing having a spiral groove that generates a dynamic pressure effect. However, there are used ceramic materials such as sintered bodies essentially consisting of conductive ceramics such as SiC, TiC, TiN, and TiB2, and composite sintered bodies between Al2O3 and TiC, and ZrO2 and NiO. There is not disclosed any of ceramic material that has characteristics intrinsic to the zirconium oxide sintered body.
Accordingly, an object of the present invention is to provide a wear resistant member for electronic equipment that can realize a stable high speed rotation when applied in the bearing ball or the like, in addition can prevent unnecessarily excessive static electricity from accumulating. Furthermore, another object is to provide a bearing and spindle motor that, by the use of such wear resistant member for electronic equipment, enable to realize higher performance and higher reliability of the electronic equipment such as magnetic recording device such as HDDs and optical disk device such as DVDs.
The wear resistant member for electronic equipment of the present invention comprises a zirconium oxide sintered body of which specific resistance is in the range from 1 to 105 xcexa9xc2x7m. In the wear resistant member for electronic equipment of the present invention, the zirconium oxide sintered body comprises a conductivity enhancer of which specific resistance is 10xe2x88x925 xcexa9xc2x7m or less.
In the wear resistant member for electronic equipment of the present invention, the conductivity enhancer contained in the zirconium oxide sintered body is preferable to be at least one kind selected from carbides and nitrides of elements of 4A, 5A, 6A and 7A groups in the periodic table, silicon and boron. In particular, at least one kind of carbide selected from tantalum (Ta), niobium (Nb), chromium (Cr), tungsten (W), molybdenum (Mo), titanium (Ti), zirconium (Zr), hafnium (Hf), manganese (Mn) and silicon (Si) carbides is preferably used.
Furthermore, in the wear resistant member for electronic equipment of the present invention, similarly with general purpose zirconium oxide sintered body, the zirconium oxide sintered body containing a rare earth compound and/or alkaline earth compound as a stabilizer can be used. In particular, it is preferable to use the zirconium oxide sintered body containing from 2 to 5 mole percent of rare earth compound together with from 0.5 to 3 mole percent of alkaline earth compound. In that case, the rare earth compound functions as the stabilizer for the zirconium oxide and the alkaline earth compound functions as sintered additive. Accordingly, the zirconium oxide sintered body of higher density can be obtained.
The wear resistant member for electronic equipment of the present invention is preferably employed in particular in electronic equipment such as magnetic recording devices such as HDDs and FDDs and optical disk devices such as CD-ROMs and DVDs. As a specific shape of the wear resistant member of the present invention, a rolling element used for a rotational driver of the electronic equipment as mentioned above can be cited. In particular, the wear resistant member of the present invention is suitable for the bearing balls for electronic equipment.
In the wear resistant member for electronic equipment of the present invention, the conductivity enhancer is preferable to have a particulate shape of a average particle diameter of 2 xcexcm or less, and furthermore a maximum diameter of the agglomerated portion of the conductivity enhancer being preferable to be 10 xcexcm or less. The conductivity enhancer is preferable to exist in the range from 3 to 20 pieces in an arbitrary straight distance of 50 xcexcm in the zirconium oxide sintered body. Furthermore, the conductivity enhancer is preferable to be only on a surface layer portion of the zirconium oxide sintered body.
In the wear resistant member for electronic equipment of the present invention, the zirconium oxide sintered body that is particularly high, among the ceramic materials, in fracture toughness and is close in the thermal expansion coefficient to metallic materials is applied. In addition to the above, to the zirconium oxide sintered body that is intrinsically an electrical insulator, the specific resistance in the range from 1 to 105 xcexa9xc2x7m is given to use. Thus, in the present invention, without damaging mechanical properties such as fracture toughness and wear resistance intrinsic to the zirconium oxide sintered body, appropriate conductivity is given. Accordingly, by applying the zirconium oxide sintered body having such properties to the bearing balls or the like, in addition to a stable high speed rotation, the static electricity affecting various adverse influences on the electronic equipment can be skillfully set free.
A bearing of the present invention comprises a bearing ball consisting of the above mentioned wear resistant member for electronic equipment of the present invention. A spindle motor of the present invention comprises the bearing of the present invention. As a specific form of the spindle motor of the present invention, a structure comprising a stator axis, a rotor, and a stator can be cited. To the stator axis, the bearing is mounted. The rotor is supported rotatable through the bearing to the stator axis and has a rotor magnet. The stator has a stator coil disposed opposite to the rotor magnet a prescribed gap apart.